Where did all oxygen in the air come from? Was
it all formed when photosynthesis started? What
was the atmosphere made of before that?

Question Date: 2001-11-13

Answer 1:

The oxygen that we currently find in the air has
two main sources. The first, and major, source is
that oxygen is liberated as a by-product of
photosynthetic chemical reactions. The second
source occurs in the atmosphere itself when water
molecules are split apart after being struck with
ultraviolet radiation. This process is called
photolysis. The molecule is broken down into one
diatomic hydrogen molecule and one free oxygen.
The hydrogen molecule is light, and has a quick
thermal velocity,so it can escape Earth's
atmosphere leaving behind the free oxygen to form
a diatomic oxygen molecule. The main source of
this atmospheric water was volcanic eruptions and
outgassing.

Basically before photosynthesis
began on large scales, free oxygen molecules in
the atmosphere were drawn down in oxidation
reactions with free metal ions on the Earth's
surface, mainly iron. Even when photosynthetic
organisms were taking control of the atmosphere,
the free oxygen molecules were being placed into
the rock record in oxidation reactions. This
forms red colored rocks, like you find in the
southwestern United States. Because of this,
early Earth had a reducing atmosphere, composed
mainly of methane,carbon dioxide, water, carbon
monoxide, and nitrogen. This would be a very
hostile place to most modern forms of life.

Answer 2:

The Earth is unusual among planets in that it has
as abundance of oxygen. However, this essential
element was not plentiful on our planet until
microorganisms known as cyanobacteria began to
produce it by photosynthesis.